Sensor elements that have at least one semiconductor component with a gas-sensitive layer are generally used to detect components of a gas mixture. Semiconductor components having a gas-sensitive layer are generally gas-sensitive field-effect transistors. In such gas-sensitive field-effect transistors the gate electrode has a coating applied to it, to which gas molecules may adsorb and thereby by way of the charge carrier density modify the characteristic curve of the transistor. That is an indication of the presence of the gas in question. The material used for the coating is one that is selective for certain particular gases that it is desired to detect. For that purpose, the coating generally, speaking contains a catalytically active material. By using different gas-sensitive field-effect transistors, each having a specific coating on the gate electrode, different gases may be detected.
Sensor elements for detecting components of gases may, for example, also be used in motor vehicle exhaust systems. Using such sensor elements, the presence of, for example, nitrogen oxides, ammonia, or hydrocarbons in the exhaust gas may be detected. However, the high temperatures of the exhaust gas from an internal combustion engine subject the sensor elements to severe stresses. In addition, particles may be contained in the exhaust gas that might abrade the coating off the gate electrode. This means that protection has to be provided for the coating on the gate electrode, but at the same time its functionality must not be impaired by such protection.
A gas-sensitive field-effect transistor having a sensitive layer having open pores is discussed, for example, in DE-A 10 2005 008 051.